Fourier-transform infrared (FTIR) spectrometers are used in analyzing and identifying the chemical composition of a sample. FTIR spectrometers typically include a Michelson interferometer having a beamsplitter, a fixed mirror and a moving mirror. The beamsplitter divides an incident IR beam into two paths, one on the fixed mirror and the other on the movable mirror, and then recombines the two beams after a path difference has been introduced by the moving mirror. A condition is thus created under which interference between the beams can occur, giving rise to intensity variations of the beam emerging from the interferometer which can be measured as a function of path difference by a detector. Because of the effect of interference, the intensity of each beam passing to the detector and returning to the source depends on the difference in path of the beams in the two arms of the interferometer. The variation in intensity of the beams passing to the detector and returning to the source as a function of the path difference provides the spectral information in the spectrometer.
The output beam is focused upon and passed through (or is reflected from) a sample, after which the beam is collected and focused onto a detector. The detector provides a time varying output signal containing information concerning the wavelengths of IR absorbance, or specular reflectance, of the sample. Fourier analysis is performed on the output signal data to yield usable information on the chemical composition of the sample.
Prior art spectrometer and interferometer combinations have suffered from various limitations. For example, precise alignment of the beamsplitter and reflectors relative to the light beam is required. Misalignment of as small as 0.1 microns will give rise to spurious interference fringes and erroneous spectral data. Detection of erroneous spectral data typically leads the investigator to check out the entire spectrometer because minor alignment corrections are not available in most current spectrometers which incorporate automatic, computer controlled positioning of the reflecting mirrors. Some prior art spectrometers include an indicator light to alert a user to a failure of the light source (typically an infrared radiation source). Failure of the infrared (IR) source and a faulty indicator can result in the spectrometer user expending considerable time troubleshooting the system when all that is required is replacement of the IR source.
The position of the beamsplitter surface between the interferometer's substrate and compensator also renders it difficult to accurately position the beamsplitter surface with respect to the fixed and movable mirrors. Heat generated by the light source and spectrometer electronics also renders optical alignment more difficult. Prior approaches for dissipating the heat have included water cooled jackets or heat sink fins attached to the light source mount which add to the complexity and cost of the spectrometer. The water cooling approach also is subject to leakage with frequently catastrophic results. Stray light from the source which is not in the beam incident upon the sample also degrades spectrometer performance. It is therefore desirable to confine the light emitted by the light source to the beam incident upon the sample and to be able to verify light source operation without trouble shooting the entire spectrometer. Finally, prior art spectrometers have suffered from overly complex optical arrangements which have reduced detected signal sensitivity and have made it difficult to isolate the optics of the spectrometer from its electronics for improved optical performance.